Description of the Prior Art
The importance of dietary fiber for use in the human and non-human system cannot be overemphasized. Dietary fiber plays a major role in health and disease resistance, physiological metabolism, and in preventive medicine. There has been considerable effort in the development of fiber-containing foods in order to benefit from the advantages of dietary fiber in the system. Further, many of these materials can be used as an effective carbohydrate and energy source in ruminant feeds. However, in order to benefit from these advantages, the lignocellulosic materials in the residues must be converted into materials which can be metabolized by the animal. Specifically, the polysaccharide portion of these agricultural residues have to be converted into monomeric sugars.
In order to accomplish this, it is important to break down the lignin in the residues to release the beneficial polysaccharides in the plant cell wall.
Originally, the delignification process used sulfuric acid and chlorine as the main agents. However, due to environmental control process problems, sulfuric acid is now being replaced with sodium hydroxide and oxygen.
As an example in woody fibers, U.S. Pat. No. 4,459,174 to Papageorges, et al. discloses a process for the delignification and bleaching of chemical and semi-chemical cellulosic pulps in which the pulp is subjected to a treatment with oxygen and subsequent treatment with peroxide. The effluent from the treatment with peroxide is at least partially recycled to the treatment with oxygen.
U.S. Pat. No. 4,451,332 to Annergren, et al. is directed to a method for the delignification of lignocellulose containing fiber material comprising mixing an oxygen-containing gas with the cellulose fiber material in order to atomize the gas and form a foam of the gas and the cellulose fiber material. This process provides a bleached, delignified cellulose fiber without bleaching the lignin substance extracted from the material.
U.S. Pat. No. 4,372,812 to Phillips, et al. is directed to a chlorine-free bleaching process for lignocellulosic pulp. This process is characterized by a series of bleaching stages comprising in sequence a peroxide bleaching stage, and at least one ozone bleaching stage.
U.S. Pat. No. 4,311,553 to Akerlund, et al. is directed to a method of producing peroxide bleached pulp by impregnating lignocellulose fiber material with an aqueous silicate solution containing a sequestering agent. The fiber material is preheated with saturated steam and defibrated between two grinding disks in an atmosphere of saturated steam at a temperature of 100.degree.-170.degree. C.
U.S. Pat. No. 4,298,425 to Ranzen, et al. is directed to a method and apparatus for producing fiber pulp of improved paper-forming characteristics from lignocellulose-containing material such as wood chips and the like.
U.S. Pat. No. 4,214,947 to Berger is directed to the treatment of a cellulosic material in the form of wood chips to produce at least partial delignification without mechanical grinding. The material is brought into contact with a reagent, e.g., steam or a chemical reagent, and is subjected to alternate increases and decreases in pressure.
U.S. Pat. No. 4,187,141 to Ahrel is directed to a method of producing mechanical pulp of improved brightness and light-scattering properties from wood chips, which are ground between a pair of disks. The chips are impregnated with a solution of alkali and introduced into a pressure vessel which is in communication with the grinding zone.
U.S. Pat. 4,444,621 to Lindahl is directed to a process and apparatus for the deresination and brightness improvement of cellulose pulp, by adding an alkali to the pulp, along with a sufficient oxidizing bleaching agent.
While the above processes are mainly directed to the delignification of woody-like materials, there are other processes known to the art which disclose the delignification of non-woody biomasses to produce food fit for human and animal consumption. For example, U.S. Pat. 4,136,207 to Bender discloses a process for the delignification and fractionation of non-woody substrates using a reactor and acid hydrolysis. This process uses a pH of 1.5 as the first step with heat and pressure and a residence time of 6-13 minutes. Hemicellulose is extracted from the residues, and the residues are subjected to hydrolysis for further fermentation to ethanol, butanol, acetic acid, furfural, and xylitol. The cellulose and lignin are then treated with an alkaline solution and separated for independent uses.
U.S. Pat. No. 4,649,113 to Gould discloses a batch process for the delignification of agricultural residues to produce cattle feeds, chemical feeds or dietary fibers through the separation of these components. The agricultural crop residues and other non-woody lignocellulosic plant substrates are treated with hydrogen peroxide at a controlled pH within the range of about 11.2 to 11.8. The substrates are partially delignified. This process does not use a reactor or mechanical shear and compression device, but utilizes pHs within the range of about 11.2 to 11.8 with hydrogen peroxide in the liquid. The cell walls are fractured in approximately 4 to 6 hours. The product can be used for animal feeds. It is also possible to separate the liquid from the cell walls if dietary fiber as a product is desired.
While there are processes and apparatuses available which delignify both woody and non-woody cellulosic materials, these processes have inherent deficiencies. For example, with the process as disclosed in the '113 patent to Gould, maximum delignification of biomass or non-woody lignocellulosic materials is achieved by the use of substantial amounts of hydrogen peroxide in an aqueous solution at a pH of about 11.5 in stored tanks for 4 to 6 hours at temperatures between approximately 50.degree. and 120.degree. F. With this process, a substantial amount of chemicals must be utilized in order to effect the required delignification of the fiber.